Ferrous alloys for high temperature service



Patented Dec. 16, 1947 FERROUS ALLOYS FOR HIGH TEMPERA- TUBE SERVICE Russell Franks and William 0. Binder, Niagara Falls, N. Y., assignors to Haynes Stellite Company, a corporation of Indiana No Drawing. Application June 13, 1945, Serial No. 599,305

8 Claims. (Cl. 75-126) This invention relates to ferrous alloys for use at high temperatures, referring more particularly to alloys suitable for use in applications where great strength at high temperatures is required.

The trend of modern engineering is toward the utilizationof high temperatures for many and diverse operations. For example, chemical processes are today conducted at very high temperatures, a notable instance being petroleum refining. Also, the quest for improvedpower sources has led to the investigation and development of such devices as superchargers, gas turbines, jet propulsion apparatus and the like, all operating at high temperatures. These developments demand of the metallurgist metals and alloys which will withstand prolonged exposure to temperatures well above about 700 F. and in many instances well above about 1200 F. The problem is complicated by the fact that severe mechanical stress is often encountered at these temperatures.

For parts of such devices as superchargers, gas turbines, jet propulsion apparatus and the like, it is necessary to employ alloys that are capable of withstanding severe mechanical stress at high temperatures. Depending upon the design and the intended use of such devices, the temperature ranges at which they operate may be separated into a range between 900 F. and about 1200 F. and into a range upwards of 1200 F. Associated parts and apparatus may be required to withstand temperatures of about 700 F. and above. In devices operating within the lower temperature range, generally much higher stresses are applied than in devices operating above 1200 F. In many instances it is desired that alloys for use in such apparatus be capable of being hot-worked and machined, while in other instances the alloys may be employed in the form of castings. In any event, the alloys must have high strength.

A number of alloys have been proposed for use at high temperatures, but the utility of these alloys has been limited either because they are not hot-workable or machinable, or because they become brittle upon prolonged exposure to high temperatures. One of the characteristics of highly alloyed ferrous materials is that as the iron-base solid solution alloy contains more and more of the alloying metals to increase high temperature strength, the stability of the materials at high temperatures tends to decrease so that on prolonged exposure to high temperatures the materials become excessively brittle.

It is the principal object of this invention to provide ferrous alloys suitable for use in applications where temperatures above about 700 F.

2 are normally encountered. A further object is the provision of hot-workable and machinable alloys for use at such elevated temperatures. Another object is the provision of alloys capable of withstanding severe mechanical stress at elevated temperatures above about 700 F. A more specific object is the provision of alloys and articles wrought or cast therefrom capable of withstanding severe mechanical stress at elevated temperatures above about 700 F. up to about 1200 F.

The invention by means of which these objects are achieved is based on the discovery that the addition of small, properly-proportioned quantities of molybdenum, tungsten and at least one element selected from the group consisting of columbium, tantalum, titanium, and vanadium to iron-chromium-cobalt alloys produces a remarkable increase in the high temperature strength of such alloys without detrimentally affecting their high temperature stability.

The invention comprises alloys containing 10% to 30% chromium, 10% to 40% cobalt, 0.5% to 7.5% molybdenum, 0.5% to 10% tungsten and 0.5% to 7% in the aggregate of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the remainder of the alloys being iron except for incidental impurities and small quantities of elements customarily present in steels of good quality. Generally preferred ranges for molybdenum and tungsten are 1% to 5% molybdenum and 0.5% to 5% tungsten. No more than 2% each of titanium and vanadium should be present in the alloys. 1

Carbon is always present in the alloys of the invention. Preferably it does not exceed about 1%, and it hot working of the alloys is desired, the maximum carbon content should be 0.35%. Nitrogen is importantly beneficial and is preferably present in a proportion up to 0.25%. Silicon and manganese may be present, the silicon content preferably not exceeding 1% and the manganese content not exceeding 2% if hot working is desired.

A useful test for determining the suitability of materials for use at high temperatures is the so-called stress-rupture test. In this test several samples of an alloy to be tested are maintained at a given temperature, each sample being subiected to a different measured stress. The time required to cause failure of the samples under these conditions of temperature and stress is determined, and the time and stress values obtained are plotted to yield a curve for the par- 3 ticular material under test be determined the stress the material will withstand for a, given period of time, say 1,000 hours, at the particular temperature for which the curve was drawn. This test provides a convenient method of'determ'ining the load carrying ability of a material. It also gives some indication of whether or not the material becomes embrittled upon prolonged exposure at the temperature of the test. A brittle material will break without elongation, whereas ductile material will elongate before failure.

Stress-rupture tests conducted in the manner just described indicate that the alloys of the invention maintain great strength at temperatures as high as 1200 F. and that even at such high temperature the alloys possess good ductility.

Typical examples or the improvement in high temperature strength of chromium-cobalt steels imparted by the addition of molybdenum, tungsten and columbium in various combinations are indicated by the data in the table below. In this table are reported the results of stress-rupture tests in which a stress of 40,000 pounds per square inch was applied to a cast sample of the steel to be tested while the sample was maintained at a temperature of 1200 F. The time in hours required for the sample to fail under these extremelysevere conditions is reported in the table.

r Table Composition-Remainder Substantially all Fe Hours 9,301- %Co %Mo %w %Ch %N 13 10 4 4 3.5 0.32 0.03 380 13 20 4 4 as 0.4 0.03 710 13 35 4 4 as 0.32 0.03 1,276 13 20 a 4 4 s 0.53 0.03 360 The data in the above table clearly illustrate the ability of the alloys of the invention to withstand .large stresses at elevated temperatures for prolonged periods of time without failure.

In manufacturing the alloys of this invention care should be taken that the composition limits set forth be closely adhered to with regard to the intended use of the alloys since material variations in the proportions of the several in gredients detrimentally affect the desired properties. For example, if the alloys are to be employed as castings, the carbon content may be as high as 1%; but if hot-workability is desired, the carbon content should be kept at a maximum of about 0.35% and preferably should not exceed Similarly, the proportions of molybdenum, tungsten, columbium, tantalum, titanium and vanadium, present in the alloys aflect hot-workability and weldability. A preferred upper limit for columbium and tantalum is 5%. Too high a proportion of any of these elements has a detrimental effect onhot-workability and weldability, particularly the latter property. Alloys within the composition limits defined may be welded readily by any of the common welding methods, for example, electric arc, oxyacetylene, submerged-melt electric welding, or solid phase pressure welding, sound, strong and tough welds being produced without undue embrittlement of weld metal or base metal, and such welds retain their toughness at elevated temperatures. However, if too high a proportion of any of these elements is present in the alloys, welds produced From this curve can a 4 usually'sufi'er from loss of toughness at elevated temperatures.

The presence of nitrogen in the alloys of the invention within the range indicated is important, nitrogen having a beneficial eflect on the high temperature stability of the alloys.

Being hot-workable, machinable, weldable, and castable, and possessing remarkable strength at elevated temperatures up to about 1200 E, the alloys of the invention are particularly well suited to use in the fabrication of articles such as parts of superchargers, gas turbines, jet propulsion apparatus and the like which are required' to withstand severe mechanical stress at elevated temperatures. Their freedom from embrittlement upon prolonged exposure at high temperatures recommends their use where dependability of operation is essential.

We claim:

1. An alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 0.5% to 7.5% molybdenum; 0.5% to 10% tungsten; an aggregate of 0.5% to 7 of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the titanium and vanadium contents each not exceeding 2%; nitrogen in an effective proportion up to 0.25%; the remainder substantially all iron and incidental impurities.

2. An alloy containing about 10% to -30% chromium; 10% to 40% cobalt; 1% to 5% molybdenum; 0.5% to 5% tungsten; an aggregate of 0.5% to 7% of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the titanium and vanadium contents each not exceeding 2%; nitrogen in an effective proportion up to 0.25%; the remainder substantially all iron and incidental impurities.

3. An alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 1% to 5% molybdenum; 0.5% to 5% tungsten; an aggregate of 0.5% to 7% of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the titanium and vanadium contents each not exceeding 2%; nitrogen in an efl'ective proportion up to 0.25%; carbon in an eifective proportion not exceeding 1%, the remainder substantially all iron and incidental impurities.

4. A hot-workable alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 1% to 5% molybdenum; 0.5% to 5% tungsten; an aggregate of 0.5% to 5% of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the titanium and vanadium contents each not exceeding 2%; nitrogen in an eifective proportion up to 0.25%; carbon in an effective proportion not exceeding 0.35%, the remainder substantially all iron and incidental impurities.

5. A cast article which in its normal use is required to withstand mechanical stress at elevated temperatures up to about 1200 E, which article is composed of an alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 0.5% to 7.5% molybdenum; 0.5% to 10% tungsten; an aggregate of 0.5% to 7% of at least one metal selected from the group consisting of columbium,

tantalum, titanium, and vanadium, the titanium quired to withstand mechanical stress at elevated temperatures not exceeding about 1200 E, which article is composed of an alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 0.5% to 7.5% molybdenum; 0.5% to 10% tungsten; 0.5% to 5% columbium; manganese in an effective proportion up to 2%; silicon in an effective proportion up to 1%; carbon in an effective proportion not exceeding 1%; nitrogen in an effective proportion up to 0.25%; remainder substantially all iron.

7. A wrought article which in its normal use is required to withstand mechanical stress at elevated temperatures up to about 1200 R, which article is composed of an alloy containing about 10% to 30% chromium; 10% to 40% cobalt; 0.5% to 7.5% molybdenum; 0.5% to 10% tungsten; an a gregate of 0.5% to 5% of at least one metal selected from the group consisting of columbium, tantalum, titanium, and vanadium, the titanium and vanadium contents each not exceeding 2%; nitrogen in an effective proportionup to 0.25%; the remainder substantially all iron and incidental impurities.

8. A wrought article which in its normal use is required to withstand mechanical stress at elevated temperatures not exceeding about 1200 F., which article is composed of an alloy containing about 10% to 30% chromium; 10% to 40% cobalt;

0.5% to 5% columbium; manganese in an eflective proportion up to 2%; silicon in an eflective proportion up to 1% carbon in an effective proportion not exceeding 0.35% nitrogen in an eflective proportion up to 0.25%; remainder substantially all iron.

RUSSELL FRANKS. WILLIAM O. BINDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Nitrogen in Chromium Steels, Electro Metallurgical Company of the Union Carbide and Carbon 1% to 5% molybdenum; 0.5% to 5% tungsten; 3e Corp., New York, 1941. 

